1. Field of the Invention
The present invention relates to a device for guiding an electron beam in a microwave source operating in accordance with the gyrotron principle.
2. Discussion of the Background
Such a device as above noted is known, for example, from the article by J. D. Silverstein et al. in Int. J. Electronics 53 (6), pages 539-546 (1982).
In a gyrotron as described in the abovementioned article, a high-energy electron base originating from an electron gun is sent through a strong magnetic field which is oriented parallel to the beam axis. The electrons then pass along spiral paths around the beam axis at a cyclotron frequency which is a function of the magnetic inductance.
If then this beam of circulating electrons crosses through a microwave resonator of suitable dimensions, electromagnetic oscillations are excited in the resonator which can be coupled out of the resonator.
For guiding the electron beam from the electron gun to the microwave resonator, which is constructed as a cavity resonator in the gyrotron, a beam duct is used which encloses the beam as an electrically conductive cylindrical surface area and, by limiting space charge effects, enables the electron beam to propagate unhindered.
The surface area, closed in the radial direction, of a beam duct according to the prior art, however, represents a wave guide. At high frequencies (of the order of magnitude of 100 GHz) and high powers (of the order of magnitude of 100 kw), the radius of the electron beam is large in comparison with the cut-off wavelength of the circular wave guide mode at the operating frequency. For this reason, unwanted wave modes can be excited in the beam duct unless suitable measures are taken for damping such modes.
As a suitable damping measure, it is known to provide stacks of annular plates along the beam axis inside the beam duct which plates consist alternately of absorbing ceramics (for example SiC-based) and highly conductive copper (see for example FIG. 5 in the initially quoted printed document).
However, such a beam duct containing stack-shaped integrated wave filters has a complicated structure and its manufacture is associated with considerable expenditure.
The gyrotron of known construction, shown in FIG. 1, for generating high-power microwaves essentially comprises an electron gun 1, a beam duct 5, a cavity resonator 6 and an output wave guide 7. The parts enumerated are housed in a vacuum chamber, not shown, and enclosed by a solenoid coil, also not shown, which generates the strong magnetic field necessary for the gyrotron effect.
The electron gun 1 emits an electron beam 2. With typically 4 mm, the diameter of the electron beam 2 is kept small for a 120 GHz gyrotron in order to achieve high efficiency in the conversion of beam energy into electromagnetic wave energy.
Due to the high space charge density inside the electron beam 2, connected with the small diameter, space charge effects such as, for example, current limiting or a lowering of the beam potential occur which limit the energy transport in the beam.
To enable the electron beam 2 to propagate unhindered from the electron gun 1 to the cavity resonator 6 in which the interaction between beam and electromagnetic field takes place, the beam duct 5, containing in its interior a stacked sequence of metal rings 3 and ceramic rings 4, is arranged between the two.
This stacked sequence represents a measure for damping unwanted wave modes which can be excited by the electron beam 2 in the closed surface area, acting as a wave guide, of the beam duct 5. The result is that only the wanted electromagnetic waves are excited in the cavity resonator 6 and are coupled out via the output wave guide 7.